Generally, a conventional vibration sensor uses a tiny spring between a conductor and a metal conducting plate so that the spring can cause the conductor to contact with the tip of a screw when the sensor is vibrated, thus forming a contact which triggers an alarm that will give a warning signal. The tiny spring so used is an ordinary spring which does not vibrate well and since contact may occur, the warning signal is not reliable enough.
In view of such a defect, the inventor has created a new structure of vibration sensor which applies a symmetrical vibrating spring in a vertical position. The further the distance from the middle of the vibrating spring, the smaller the diameter. Whenever the vibrating spring vibrates upon any external force, the hopper conductor above the vibrating spring moves evenly without any false signal so that a proper alarm signal can be given.